1. Field of the Invention
The present invention concerns a power laser chain, for example of the carbon dioxide type, for providing infra-red light pulses having a wavelength of 10.6 microns.
2. Description of the Prior Art
Such a chain conventionally comprises the following elements arranged in series, following the path of the light beam:
A laser oscillator for forming the beam into pulses; PA1 A modulator for shortening the pulses, that modulator comprising, at its output, a linear polarizer; PA1 A quarter wave plate suitably arranged fo transforming the linear polarization of the pulses into circular polarization; PA1 One or several laser amplifying elements for amplifying the light pulses which can subsequently be focussed on a target. PA1 A laser oscillator forming the said beam; PA1 A first linear polarizer; PA1 A first transformer of the type transforming a linear polarization into a circular polarization and vice-versa; PA1 and a laser amplifying chain, characterized in that said amplifying chain is divided into a first and a second amplifying element and comprises, moreover, in series on the path of the said beam and between those two amplifying elements: PA1 a second transformer of the said type; PA1 a second linear polarizer crossed with the first; PA1 and a third transformer of the said type.
The function of the quarter wave plate is to protect the modulator from return pulses, which result from the retrodiffusion or reflecting of a part of the light received by the target. Indeed, the pulses transmitted by that plate towards the target through the amplifying elements are polarized circularly and the same is therefore the case with the pulses retrodiffused by the latter. After having been re-amplified by the elements, these retrodiffused pulses cross through the same quarter wave plate again in the opposite direction (return direction), this transforming their circular polarization into rectilinear polarization, with a polarization direction perpendicular to that which they had on the outgoing trajectory (go direction), at the output of the modulator. The linear polarizer therefore stops them, this ensuring the protection of the modulator.
Such a chain is, for example, described in French patent No. 1 585 293, for a "Non-return device for firing at a target by a laser," in the name of Commissariat a l'Energie Atomique. It has a disadvantage when the energy of the pulses supplied is required to be increased by increasing the gain of the amplifier elements: if that gain exceeds a critical value, of 60 dB, for example, the inevitable presence of spurious reflecting or retrodiffusing surfaces at both ends of the amplifying chain causes the appearing of spontaneous laser oscillations which can be a great hindrance.
The aim of the present invention is to produce a power laser chain having an increased gain without any danger of the appearing of spontaneous oscillations.